Low pressure casting is a method of filling the mold cavity with liquid metal under pressure to form castings. Because the pressure used is low, it is called low-pressure casting. The process (see Figure 1) is: in a sealed crucible (or sealed tank) is filled with dry compressed air, the liquid metal 2 rises along the riser 4 under the action of gas pressure, enters the cavity 8 smoothly through the gate 5, and maintains the gas pressure on the liquid level in the crucible until the casting is completely solidified. Then, release the gas pressure on the liquid level, so that the non solidified liquid metal in the opening pipe flows through the crucible, and then open the mold and push it by the cylinder 12 Casting.

1 holding furnace 2 liquid metal 3 crucible 4 liquid riser 5 gate 6 sealing cover 7 lower type 8 cavity 9 upper type 10 ejector rod 11 rod plate 12 air cylinder 13 asbestos sealing gasket

The unique advantages of low pressure casting are shown in the following aspects:

1. Liquid metal filling is relatively stable;
2. The casting has good formability, which is conducive to the formation of castings with clear outline and smooth surface, and is more conducive to the formation of large thin-wall castings;
3. The casting has dense structure and high mechanical properties;
4. The process yield of liquid metal is improved. Generally, no riser is required, so that the yield of liquid metal is greatly improved, and the yield is generally up to 90%.

In addition, good working conditions; The equipment is simple and easy to realize mechanization and automation, which is also the outstanding advantage of low-pressure casting.

2 low pressure casting process design

There are two kinds of mold used in low pressure casting, metal mold and non-metal mold. Metal mold is mostly used for large-scale and mass-produced non-ferrous metal castings, while non-metallic mold is mostly used for single piece and small batch production, such as sand mold, graphite mold, ceramic mold and molten mold shell, which can be used for low-pressure casting, and sand mold is more used in production. However, the air permeability and strength of the molding material of sand mold for low-pressure casting should be higher than that of gravity casting. The gas in the mold cavity is discharged by the exhaust duct and sand pores.

In order to make full use of liquid metal feeding castings from bottom to top under pressure during low-pressure casting, it should be considered in process design

The part of the casting away from the gate shall be solidified first, and the gate shall be solidified finally, so that the casting can be fed through the gate during the solidification process to realize sequential solidification. The following measures are often taken:

1. The gate is set at the thick wall part of the casting, and the thin wall part is far away from the gate;
2. Adjust the wall thickness of the casting with processing allowance to adjust the directional solidification of the casting;
3. Change the cooling conditions of castings.

For castings with large wall thickness difference, when it is difficult to obtain the conditions of sequential solidification with the above general measures, some special methods can be adopted, such as local cooling at the thick wall of the casting, so as to realize sequential solidification.